In a communication network where a plurality of transmission apparatuses are provided, there may be a plurality of communication paths connecting between the transmission apparatuses. In such network, when the path communication quality is changed due to an occurrence of failure or the like, it is required to switch to a better path. In a case of optical communication, it is possible to switch to a proper path by using a routing technique such as GMPLS (Generalized Multi-Protocol Label Switching).
In the meantime, as a technique for monitoring the state of communication paths, there is a technique depicted in Patent Document 1 described later, for example. In Patent Document 1, depicted is a technique which periodically transmits a monitoring signal to an optical path from a transmitter side to a receiver side and monitors the state of the path.    Patent Document 1: Japanese Unexamined Patent Publication 2003-060588